Metaphors can be dangerous things. Just recently I got into a Twitter conversation with someone who was using the metaphor "it's like leaving your car unlocked, or your front door" - meaning something you would never do and would expect to be dangerous. But in my neck of the woods, that metaphor triggers different neurons, having a meaning more like something you regularly do and would never expect to be a problem. (I not only never lock my car in my or a neighbour's driveway, I also know many people who would leave their keys in their cars in someone else's driveway.) It doesn't really matter whether you think door locking is normal, the point is your metaphor needs to have the same meaning for your audience as it does for you.

For an example of a metaphor landing really badly, check out Scott Berkun's blog post on the "periodic table of visualization techniques." Now unlike Scott (who thinks the periodic table is obscure, complex, and unfondly remembered) I really like the periodic table. I think it carries a tremendous amount of important information in a very compact way, and explains some relationships succinctly. But I think it makes a poor metaphor when trying to discuss all the different ways you can present information visually. Plus, their particular version of it doesn't seem to have any actual periodicity, anything that's the same in each column, anything that's the same in each row, or any reason for the lengths of the rows. Always understand a metaphor before you use it. Otherwise you're working against your own goal - helping someone else understand your point.

Kate